Quadrupole mass filters with introductory ion accelerating field proportional to thequadrupole electric field



Feb. 15, 1966 w. M. BRUBAKER 3,235,724

QUADRUPOLE MASS FILTERS WITH INTRODUCTORY ION ACCELERATING FIELD PROPORTIONAL TO THE QUADRUPOLE ELECTRIC FIELD Filed Oct. 2, 1962 2 Sheets-Sheet l moo-2200 44 Pf j,

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QUADRUPQLE MASS FILTERS WITH INTRODUCTORY ION ACCELERATING FIELD PROPORTIONAL TO THE QUADRUPOLE ELECTRIC FIELD Filed Oct. 2, 1962 2 Sheets-Sheet 2 INVENTOR. W/uo/v M 5,?uBA/1A'R BY ZZZZ.

ATTORNEYS:

United States Patent ()fifice 3,235,724 Patented Feb. 15, 1966 This invention relates to a quadrupole mass filter and more particularly, to aqu-adrupole mass filter wherein the energy of the entering ions to the filter is proportional to the voltage of excitation thereon.

Mass filters present a means of investigating a beam of ions by sorting the ions into a spectrum according to their mass to charge ratio (M/e) and recording the relative abundance of those under investigation. This principle is well known as adapted to mass spectrometers which employ a magnetic and an electrostatic field to sort the ions according to their mass to charge ratio. A variation of this apparatus for separating ions is known as a quadrupole mass filter.

Quadrupole mass filters present a means of separating or selectively detecting ions having particular mass to charge ratios much in the same manner as more conventional mass spectrometers. However, the quadrupole filter accomplishes detection or separation in a unique way, without a magnetic field, by utilizing the motion of charge particles through an electric field having both alternating and static components.

A quadrupole mass filter consists primarily of four electrodes connected symmetrically about a central axis. The electrodes ideally consist of four identical hyperbolae, but practically shaped as cylindrical rods, with the potential of neighboring electrodes being equal in amount but op posite in sign. The potential consists of a DC. component and a superimposed radio frequency voltage such that an ion entering along the axis of the electrodes may either strike an electrode or pass through the filter. Generally, the path the ion will take depends on its mass and on the frequency and the amplitude of the voltages applied to the quadrupole electrodes. When the electrical fields are, as previously described, an ion having a stable trajectory moves in bounded volume about the axis to a receiving collector located near the exit portion of the electrodes. Ions with unstable trajectories will normally divert from the axis and eventually strike the electrodes thereby being separated from the desired stable ions.

A theoretical explanation for the operation of quadrupole mass filters is clearly set forth in co-pendin'g application serial No. 158,697, now US. Patent No. 3,129,327, by Wilson M. Brubaker, filed December 12, 1961.

One of the necessary conditions for separating ions of mass m from ions of mass (m-l-Am) is that the ions experience a minimum number of radio frequency cycles as they pass through the filter. In particular, it has been found that n25(m/Am) where n is the number of cycles impressed on the assembly during the passage of the ions, and

where l is the length of the filter, v is the axial velocity of the ion, and f is the frequency of excitation of the unit.

The y-component of the velocity of the ions is given by:

where V e is the energy associated with the radial component of the velocity, V, is proportional to the potential V through which the ions are accelerated. At constant frequency the conditions of operation of the filter are:

quadrupole excitation voltage) V /m: constant combining Equations 1, 2, and 3, the axial relations conducive to constant resolution are given by:

and comparison with Equation 4 yields the result that V /V is constant and analysis of the operation of the filter shows that the condition necessary for successful operation is when:

V /V 20.064(dm/m) (6) Accordingly, it has now been realized that if mass scanning is by voltage variation, at constant frequency, and in proportion to the energy of entering ions, a quadrupole mass filter results in which the transmission efficiency remains approximately similar for ions of all masses.

Generally described, this invention provides a quadrupole mass filter having means for providing entering ions with equal energy, and means for proportionalizing the entering energy of the ions with the voltage Otf excitation applied to the primary electrodes of the mass filter.

More particularly, this invention provides a multi-pole mass filter having a plurality of primary electrodes spaced about a central axis, and having means for applying a DC. voltage to said electrodes to produce a static multipole electric field component between the primary electrodes and mean-s for applying an AC. voltage to the primary electrodes to produce an alternating multipole electric field component between the primary electrodes. In combination with this filter structure is a means for providing energy to entering ions proportional to the voltage of excitation applied to the primary electrodes such that the transmission efficiency of ions of all masses will be approximately similar.

This invention will be further described with reference to the drawings in which:

FIG. 1 is a general perspective view of a quadrupole mass filter;

FIG. 2 is a more detailed schematic side elevation taken along the Y-Z plane of FIG. 1 and including the circuit according to this invention; and

FIG. 3 is a schematic diagram indicating the general manner in which voltages are applied to quadrupole electrodes.

Referring to FIGS. 1 and 2, the quadrupole mass filter includes a cylindrical metallic housing 10 having four primary electrodes 12, 14, 16 and 18, mounted therein on electrical insulating supports 20. The primary electrodes are in the form of coextensive conductive cylindrical rods extending parallel to one another and disposed symmetrically about the central axis Z of the filter. One pair of diametrically opposed rods 12 and 14 lie with their centers in the Y-Z plane and will hereinafter be referred to as Y poles; and the other pair of opposed rods designated 16 and 18 lie with their centers in the X-Z plane and will be referred to hereinafter as X rods. A conductive plate 22 is mounted across one end of the housing and has a centrally located circular aperture 24 forming the entrance of the filter and called the ion entrance aperture. At the opposite end of the filter is mounted a conductive plate 26 having a central circular aperture 28 I therein which serves as the ion exit aperture for the filter.

An ion source is provided near the aperture 24. It consists of an electron beam between a. repeller 34 and an ion accelerator grid 36. The electron flows from the filament 30 to the anode 32, approximately equidistant from the repeller and the ion accelerator. The repeller is connected by lead wire 38 to resistor R which is further connected by line 39 to an oscillator and rectifier system 40, shown in greater detail in FIG. 3, which provides voltage to quadrupole electrodes 12 and 14 by lines 42 and 44 and to electrodes 16 and 18 by lines 43 and 45. The system is grounded by lines 41. A scanning D.C. power supply 44 is also included for energizing the oscillator and providing the accelerating potential for the ions.

The ion accelerating grid 36 located near the aperture 24 is connected by line 50 to one end of a resistor R the other end of which is connected to the ion repeller plate by line 38. A third resistor R is included which connects the ion accelerator line 50 to ground line 42..

The housing is enclosed by a conductive rear wall 52 on which anelectrical insulating support 54 is mounted. An ion collector 56 is mounted on support 54 opposite the ion exit aperture 28. Ions entering the entrance aperture 24 from the source and traversing the filter impinge upon collector 56 and thereby produce an electrical charge which is measured by any conventional measuring circuit 58 connected between the collector and a ground.

When used in the laboratory the housing 10 is evacuated and the ion source is mounted over the entrance aperture. When the mass filter is employed for upper atmosphere research, the ion entrance aperture may be exposed and the vacuum inside is provided by the vacuum of space.

In an actual device fabricated according to the illustrations in FIGS. 1 and 2, the interior of the housing and its contents are gold plated, including the primary electrodes, but not including the insulating supports for these electrodes.

The electrical connections are indicated in general fashion in FIGS. 1 and 2. Diametrically opposed rods are connected together in pairs, and both AC. and D.C. voltages (V -l-V are applied between the two rod pairs. The X-rods 16, 18 are D.C. positive, and the Y-rods 12, 14 are D.C. negative. This creates a quadrupole electric field having both AC. and D.C. components between the rods.

A more detailed circuit for applying and controlling the voltages on the primary electrodes, particularly as to the apparatus illustrated in FIGS. 1 and 2, is shown in the circuit diagram of FIG. 3. In this circuit a radio frequency generator 56, the peak output voltage of which and the frequency of which is adjustable, supplies an AC. signal through capacitive coupling across the pairs of primary electrodes 12, 14 and 16, 18.

Full wave rectification of the A.C. signal from the radio frequency generator is provided by a pair of diodes 58, 60, and the resulting D.C. voltage is applied across the terminals 62, 64 of a voltage divider network. At terminal 62 the D.C. voltage is maintained positive with respect to ground. At terminal 64 the voltage is maintained negative with respect to ground. A balance adjustment is provided by a resistor 66 having an adjustable grounded tap.

The D.C. potential to be applied across the pairs of primary electrodes is derived over a pair of leads 67, 68 from a first arm of the divider network. This D.C. potential may be adjusted by a variable shunt resistor 70.

The other two arms of the divider network each contain crossed potentiometers with ganged taps 76 and 78, respectively, the taps being coupled to the electrodes for maintaining a D.C. voltage across electrode pairs.

Typical values for the circuit components are indicated on the drawing. Typical operating frequencies and voltages are as follows:

Frequency kc 781 Primary electrodes:

V v 48.2 V v 282 In the XZ plane, the D.C. potential on the X-rods repel positive ions toward the filter axis; whereas, the A.C. potential causes positive ions to oscillate about the filter axis, analogous to a resonant system. If the amplitude of the oscillation becomes too large, the ion will be lost. It should be noted that while in the usual resonant sytsem the amplitude is bounded for excitation frequencies on either side of resonance, in the quadrupole system, the ions appear to oscillate with a bounded amplitude only when their mass is above the resonant mass. For the resonant mass and all lighter masses, the amplitude appears to increase without limit.

In the Y-Z plane, the positive ion is pulled outwardly from the axis by the negative D.C. potential on the Y- rods. If the trajectory of the ion is to be stable, it must result from the AC. field whereby the motion of the ion in a non-uniform electric field causes the net momentum impulse toward the axis to equal or exceed that directed away from the axis when the ion approaches the outer limits of a stable trajectory. This condition will obtain only when the ion has a .sufiicient low mass so that it moves enough. For higher masses, the ion will not move enough responsive to the AC. field to achieve stability, and will be lost from the beam.

Thus, in the X direction positive ions are stable in the D.C. fields and the influence of the AC. field is to make them unstable. The lighter ions tend to be unstable. In the Y direction, stability results solely from the motion of the ions in the non-uniform A.C. field, and the heavier ions tend to be unstable.

According to the theory of operation of the present invention, the transmission eificiency for those ions within the pass band of the filter is the same regardless of the mass of the ions.

This may be accomplished by providing an accelerating force for the ions (in the source) which is proportional to the excitation potential of the rods. By further providing a scanning means by the voltage of excitation, at constant frequency, a quadrupole mass filter may be realized having the transmission efficiency independent of mass.

In operation of a quadrupole mass filter, it is desirable that the transmission and resolving power remain independent of mass. When mass scanning is by voltage variation and at constant frequency, mass independency may be accomplished by causing the energy with which the ions enter the filter to be proportional to the voltage of excitation of the filter.

After examination of the axial and radial conditions influencing the transmission and resolution of a mass filter, the desirability of having the injection energy of ions proportional to the voltage of excitation of the mass filter may be readily realized. This is accomplished by having the injection voltage proportional to the mass of the ions which reach the collector.

Numerous variations will readily be apparent to those skilled in the art. Accordingly, it is intended that the invention be not limited to that specifically shown, and that the appended claims are contemplated to cover all modifications falling within the scope and spirit of the invention.

What is claimed is:

1. In a multi-pole mass filter having a plurality of substantially parallel primary electrodes spaced symmetically about a central axis, means for imposing an excitation voltage on the primary electrodes including means for applying a DC. voltage to produce a static multi-pole electric field component between the primary electrodes and means for applying an A.C. voltage to produce an alternating multi-pole electric field component between the primary electrodes, a source of ions located outside the mnlti-pole field, and propelling electrode means located in the region of the source of ions, the improvement which comprises means for applying a potential to the propelling electrode means for injecting ions from the source into the mass filter and means for maintaining the potential applied to the propelling electrode means uniformly proportional to the multi-pole electric field to cause the energy with which the ions are injected into the mass filter to be uniformly proportional to the excitation voltage.

2. In a multi-pole mass filter having at least four approximately parallel primary electrodes spaced diametrically opposite one another in pairs about a central axis, means for applying an excitation voltage to the primary electrodes including means for applying a DC. voltage of one polarity to one pair of diametrically opposite primary electrodes and a DC. voltage of opposite polarity to another pair of diametrically opposite primary electrodes so as to produce a static multi-pole electric field component between the primary electrodes and means for applying an A.C. voltage to the primary electrodes to produce an alternating multi-pole electric field component between the primary electrodes and an entrance aperture and an outlet aperture for the mass filter, an ion source located adjacent the entrance aperture, the source havingan ion repeller electrode and an ion accelerating electrode, means for introducing ions into a space between the repeller and accelerating electrodes, the improvement comprising means for imposing an accelerating electric potential between the repeller and accelerating electrodes to inject the ions through the entrance aperture along the central axis toward the outlet aperture and means for causing the accelerating potential to vary directly and uniformly proportional to the excitation voltage whereby the transmission efiiciency of ions passing through the filter are similar and independent of ion mass.

3. In a multi-pole mass filter having at least four approximately parallel primary electrodes spaced diametrically opposite one another in pairs about a central axis, means for applying an excitation voltage to the primary electrodes including means for applying a DC. voltage of one polarity to one pair of diametrically opposite primary electrodes and a DC. voltage of opposite polarity to another pair of diametrically opposite primary electrodes to produce a static multi-pole electric field component between the primary electrodes and means for applying an A.C. voltage to the primary electrodes to produce an alternating multi-pole electric field component between the primary electrodes, and an entrance aperture and an outlet aperture for the mass filter, and an ion source located adjacent the entrance aperture, the source having an ion repeller electrode and an ion accelerating electrode, and means for introducing ions into a space between the repeller and accelerating electrodes, the improvement comprising first impedance means connecting the accelerating electrode to the excitation voltage, second impedence means connecting the repeller electrode to the excitation voltage, the first and second impedence means being chosen such that the electric poten tial imposed between the repeller and accelerating electrodes is maintained uniformly proportional to the excitation voltage whereby the transmission efficiencies of ions passing through the filter are similar and independent of ion mass.

References Cited by the Examiner UNITED STATES PATENTS 2,764,691 9/ 1956 Hipple 25041.9 2,939,952 7/1960 Paul 25041.9

FOREIGN PATENTS 1,230,714 9/1960 France.

MLPH G. NILSON, Primary Examiner. 

1. IN A MULTI-POLE MASS FILTER HAVING A PLURALITY OF SUBSTANTIALLY PARALLEL PRIMARY ELECTRODES SPACED SYMMETICALLY ABOUT A CENTRAL AXIS, MEANS FOR IMPOSING AN EXCITATION VOLTAGE ON THE PRIMARY ELECTRODES INCLUDING MEANS FOR APPLYING A D.C. VOLTAGE TO PRODUCE A STATIC MULTI-POLE ELECTRIC FIELD COMPONENT BETWEEN THE PRIMARY ELECTRODES AND MEANS FOR APPLYING AN A.C. VOLTAGE TO PRODUCE AN ALTERNATING MULTI-POLE ELECTRIC FIELD COMPONENT BETWEEN THE PRIMARY ELECTRODES, A SOURCE OF IONS LOCATED OUTSIDE THE MULTI-POLE FIELD, AND PROPELLING ELECTRODE MEANS LOCATED IN THE REGION OF THE SOURCE OF IONS, THE IMPROVEMENT WHICH COMPRISES MEANS FOR APPLYING A POTENTIAL TO THE PROPELLING ELECTRODE MEANS FOR INJECTING IONS FROM THE SOURCE INTO THE MASS FILTER AND MEANS FOR MAINTAINING THE POTENTIAL APPLIED TO THE PROPELLING ELECTRODE MEANS UNIFORMLY PROPORTIONAL TO THE MULTI-POLE ELECTRIC FIELD TO CAUSE THE ENERGY WITH WHICH THE IONS ARE INJECTED INTO THE MASS FILTER TO BE UNIFORMLY PROPORTIONAL TO THE EXCITATION VOLTAGE. 